Effect of the stacking order, annealing temperature and atmosphere on crystal phase and optical properties of Cu2SnS3

Direct vapour transport grown Cu2SnS3 crystals: exploring structural, elastic, optical, and electronic properties

Copper tin sulphide (Cu2SnS3) (CTS) has emerged as a potent material for applications in photovoltaic, thermoelectric, electrochemical, biological, and other fields. CTS has superior properties such as non-toxicity, direct bandgap, p-type conductivity, variable crystal structure, alterable morphology and ease of synthesis, and it is a better substitute for conventional semiconductor materials. In the present work, CTS crystals were grown using direct vapour transport. Investigation through X-ray diffraction showed that the as-grown CTS crystals possessed a cubic unit cell structure with a = b = c = 5.403 Å. The analysis of the binding energies and composition of constituents of the as-grown CTS crystals via X-ray photoelectron spectroscopy confirmed the presence of Cu1+, Sn4+ and S2-. The experimental bandgap of CTS crystals is 1.23 eV, which was confirmed by diffuse reflectance spectroscopy. The investigation of elastic, optical, thermal and electronic properties of CTS crystals was carried out via density functional theory employing generalized gradient approximation with the Perdew-Burke-Ernzerhof exchange-relationship functional. The first-ever analysis of the temperature-dependent elastic properties of CTS crystals revealed greater stability at elevated temperature (953 K). Dielectric properties, reflectivity, refractive index, loss function, extinction and absorption coefficients of CTS crystals were computed and analyzed in detail. The evaluation of the electronic band structure with density of states revealed valence band maximum and conduction band energy level contributions, showing a bandgap of 1.2 eV. The obtained results are discussed in detail.

A Two-Step Magnetron Sputtering Approach for the Synthesis of Cu2ZnSnS4 Films from Cu2SnS3\ZnS Stacks

Cu₂ZnSnS₄ (CZTS) is regarded as one of the emerging materials for next-generation thin film solar cells. However, its synthesis is complex, and obtaining a single-phase CZTS thin film is difficult. This work reports the elaboration of Cu₂ZnSnS₄ thin films by a sequential magnetron sputtering deposition of Cu₂SnS₃ (CTS) and ZnS as stacked films. Initially, the CTS films were prepared on a soda lime glass substrate by annealing Cu and SnS₂ stacked layers. Second, ZnS was deposited by magnetron sputtering on the CTS films. The CTS\ZnS stacks were then annealed in Sn + S or S atmospheres. The tetragonal CZTS structure was obtained and confirmed by grazing incidence X-ray diffraction and Raman spectroscopy. The morphological and compositional characteristics, measured by scanning electron microscopy and energy-dispersive spectroscopy, revealed large grains and dense surfaces with the elemental composition close to the intended stoichiometry. Additional X-ray photoemission spectroscopy measurements were performed to determine the surface chemistry and particularities of the obtained films. The optical properties, determined using conventional spectroscopy, showed optimal absorber layer band gap values ranging between 1.38 and 1.50 eV. The electrical measurements showed that all the films are p-type with high carrier concentrations in the range of 10¹⁵ to 10²⁰ cm^-3. This new synthesis route for CZTS opens the way to obtain high-quality films by an industry-compatible method.